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Student Examines Material
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Dr. Patrick Shambeger 

The central premise underlying this REU project is that a fundamental understanding of heterogeneous nucleation is required to predict and control solid-state phase transformation behavior at small length scales. Diffusionless solid-state phase transformations generally nucleate from sparse, high energy atomic-scale defects in the crystal lattice14-16.As the volume of material undergoing transformation decreases, the probability of potent nucleation sites occurring within that volume decreases, resulting in 1) unpredictable transformation, 2) increased transformation hysteresis and, in extreme cases, 3) complete quenching of the phase transformation17-29.This phenomenon results in a critical length scale, Lcr, below which nucleation becomes statistically unlikely, where Lcrdepends on both the magnitude of the energy barrier to nucleation, as well as the population of defects within the volume available to serve as nucleation sites. Intentional introduction of defects can serve to increase nucleation sites, but can also impede the motion of mobile interphase boundaries (and hence, the growth of new phase domains). This topic strongly integrates with other topics on multifunctional alloys, including SMAs, which are dependent on diffusionless solid-state phase transformations.

Research Plan:
REU students will investigate reversible thermoelastic martensitictransformations (MTs) in different representative model systems, including 1) magnetocaloric effect materials (e.g., (Fe,Mn)2(P,Si)), 2) SMAs (e.g., Ni-Fe-Mn-Ga), and 3) metal-insulator transitions (e.g., VO2Fig.1)30-32.Students will use chemical or irradiation techniques to intentionally introduce controllable defect populations into a particular material system, and will characterize resulting phase transformation behavior and nucleation statistics by either direct techniques (e.g., optical microscopy, atomic force microscopy), or by indirect measures (e.g., calorimetry, electrical resistance). Students will present regular research updates in Dr. Shamberger’s research group meetings, and will develop a broad appreciation for both classical nucleation and growth theory, as well as statistical analysis necessary to interpret collective transformation behavior resulting from a large number of nucleation events.